Section 4. For purposes of this chapter, a “trench” shall be defined as an excavation which is narrow in relation to its length, made below the surface ground in excess of 3 feet below grade and the depth of which is, in general, greater than the width, but the width of the trench, as measured at the bottom, is no greater than 15 feet and the words “ excavator”, “excavation” and “emergency” shall have the same meanings as defined in section 40 of chapter 82.